The present invention relates generally to sewing machines, and in particular to a speed setting arrangement for sewing machines.
The operating speed of sewing machines is usually controlled with respect to a reference setting speed. Conventional reference speed setting arrangments comprise a set of permanent magnet mounted for rotation with the foot pedal and a magnetic sensor stationarily mounted with respect to the magnet. However, since the signal provided by the magnetic sensor represents the distance to the magnet and since the magnet has a temperature dependent characteristic, the sensor's output signal tends to vary not only as a function of distance to the magnet but also as a function of ambient temperature. The intensity of the magnet also vary as a function of time, or ageing, so that the signal would cease to be a valid indication of the amount of pedal depression. Due to the inherent inaccuracy, the conventional speed setting arrangement additionally requires an optoelectrical device formed by a set of light emitting and receiving elements, and a light intercepting plate for the purpose of generating start-stop signals. The latter is arranged to move with the foot pedal for intercepting the path of light emitted from the light emitting element to the receiving element. This adds to the sewing machine cost and further degrades the reliability of the sewing machine. The inaccuracy of the conventional speed setting arrangement is particularly disadvantageous for digitally processing the speed of sewing machines.